Metal detection and alarm systems

ABSTRACT

A metal detection alarm system detects the presence of an article of a particular metal in a given region by measuring the disturbance of two oscillating electromagnetic fields, one oscillating at from 5 to 20 kHz., the other at from 16 Hz. to 5 kHz., and also of the disturbance of a magnetic field established by D.C. pulses produced at a rate of 0 to 16 pulses per second. Receivers tuned to respond to the electromagnetic and magnetic fields established in the region produce a different response when a metal is present, the change in responses depending upon different parameters of the metal article. The outputs from the receivers are compared with the transmitted signals or in a logic circuit with standard signals and if at least two of the receiver outputs indicate the presence of the particular metal, an alarm is actuated. Applied to revolving doors, apparatus to prevent a person carrying a firearm into a bank or other building, or from entering an aircraft, is obtained. The system can also be used in conjunction with conventional non-revolving doors or with windows. A temperature controller utilizing the change of resistance with temperature of a thermistor is also described. The thermistor is located in the base circuit of a transistor which, in turn, is in a control circuit which determines the length of pulses applied to the gates of two thyristors or silicon controlled rectifiers. The time of switching the thyristors or S.C.R.s and length of pulse controls the supply of power to a heater.

Brown 51 Oct. 10, 1972 [54] METAL DETECTION AND ALARM SYSTEMS [72]Inventor: Yul] Brown, 182 Auburn Road, Au-

burn, New South Wales, Australia [22] Filed: June 22, 1970 [21] Appl.No.: 47,974

[30] Foreign Application Priority Data June 26, 1969 Australia..57077/69 June 26, 1969 Australia ..57078/69 July 11, 1969 Australia..57800/69 Aug. 6, 1969 Australia ..59123/69 Dec. 22, 1969 Australia..65563/69 Jan. I2, 1970 Australia ..PA 0058 [52] U.S. Cl. ..340/258 R,340/258 C, 340/258 D,

' 324/41 [51] Int. Cl. ..G08b 13/14 [58] Field of Search ..340/258 R,258 C, 258 D; 324/41 [56] References Cited UNITED STATES PATENTS3,582,931 6/l97l Nztwrocki ..340/258 D Primary Examiner.lohn W. CaldwellAssistant ExaminerMichael Slobasky Attorney-Stevens, Davis, Miller &Mosher l 1 HFO 2 I 7 LFO ABSTRACT A metal detection alarm system detectsthe presence of an article of a particular metal in a given region bymeasuring the disturbance of two oscillating electromagnetic fields, oneoscillating at from S-to 20 kHz., the other at from 16 Hz. to 5 kHz.,and also of the disturbance of a magnetic field established by DC.pulses produced at a rate of 0 to 16 pulses per second. Receivers tunedto respond to the electromagnetic and magnetic fields established in theregion produce a different response when a metal is present, the changein responses depending upon different parameters of the metal article.The outputs from the receivers are compared vwith the transmittedsignals or in a logic circuit with standard signals and if at least twoof the receiver outputs indicate the presence of the particular metal,an alarm is actuated. Applied to revolving doors, apparatus to prevent aperson carrying a firearm into va bank or other building, or fromentering an aircraft, is obtained. The system can also be used inconjunction with conventional non-revolving doors or with windows. Atemperature controller utilizing the change of resistance withtemperature of a thermistor is also described. The thermistor is locatedin the base circuit of a transistor which, in turn, is in a controlcircuit which determines the length of pulses applied to the gates oftwo thyristors or silicon controlled rectifiers. The time of switchingthe thyristors or S.C.R.s and length of pulse controls the supply ofpower to a heater.

14 Claims, 7 Drawing Figures PATENTEBOBT 10 um- I 3.697.972 sum 1m 4 LFOLFR 52 FIGURE I 9 a FIGURE 4 PATENTEDocT 10 1912 Y 3.697.972

SHEET 3 OF 4 FIGURE 3 FIGURE 6 RI R2 I ALARM PATENTEDncno m2 SHEET '4 OF4 m umDmvI METAL DETECTION AND ALARM SYSTEMS This invention relates toalarm systems and more particularly to alarm systems actuated by a metalobject, especially steel. The detection system utilizes the effects onelectromagnetic and magnetic fields of the surface, mass and quality ofa metal.

The present invention has been developed as a consequence of the need toprovide, in certain instances, a

warning of a possible armed attack. Two particular uses of theinvention, in banks and at airports, spring immediately to mind. Theneed for adequate alarm systems for banks is one well recognized at thepresent time, when scarcely a day passes without an armed hold-up beingreported. Airport security has also featured prominently in thenewspapers recently for there is a clear need to provide a means wherebypersons carrying firearms can be prevented from entering an aircraft,for firearms, if carried aboard, are available for use in acts of piracyin the air. These acts of piracy are not uncommon and are often referredto as hijackings. The present invention is not, however, limited tothese two uses but can be incorporated into any location where it wouldbe advantageous to detect the presence of a person carrying a firearm.

Another need, namely the desirability of restraining a would-be armedrobber or hijacker with a view to his subsequent arrest, also promptedthe development of aspects of the present invention.

It is an object of the present invention to provide an alarm systemwhich will respond to the presence of a metallic object, particularlysteel.

According to the present invention, an alarm system comprises a regionthrough or over which a person carrying a metallic object must pass; afirst and second oscillator coupled, respectively, to a first and secondtransmitting coil, each said coil being adapted to produce signalswithin said region; a first and second receiving coil each responsive tosignals within said region and coupled to a first and second receiver,respectively; said first oscillator being adapted to produce a lowaudio-frequency signal, said second oscillator being adapted to producea high audio-frequency signal, said first and second receivers beingtuned to the frequency of the signals of said first and secondoscillators, respectively; a pulse generator adapted to supply D.C.pulses to a magnetic field producing coil to produce a magnetic fieldwithin said region, and a third receiving coil coupled to a magneticflux detector; each of said receivers and said magnetic flux detectorbeing adapted to produce an output signal to be either fed into a logiccircuit capable of comparing each said output signal with a respectivereference signal or used to actuate a switching device; said logiccircuit producing a signal when there is a departure of at least two ofsaid output signals from their respective reference signals or saidswitching devices providing an output signal on actuation of at leasttwo of said switching devices, said signal from said logic circuit orsaid switching devices causing actuation of an alarm.

Embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating the general inter-relationship ofelements included in an alarm system,

FIG. 4 illustrates a typical arrangement (not to scale) A for an alarmsystem, constructed in accordance with the present invention, at anairport,

FIG. 5 is a circuit diagram of-a constant temperature control circuitthat may be used with embodiments of the present invention, and

FIG. 6 is a schematic representation of receiving aerials in a casewhere two receiving aerials are used, on opposite sides of a regionunder surveillance.

The present invention uses the fact that a metal object disturbeselectromagnetic, and magnetic fields in a manner dependent upon, interalia, the actual metal of which the object is constructed. For example,a ferromagnetic material has a magnetic permeability which is very muchgreater than that of pure brass, consequently the effect of introducinga ferromagnetic material into a magnetic field is much more pronouncedthan the effect of introducing a pure brass object into the samemagnetic field. High audiofrequency (in the range 5 kHz. to 20 kHz.)electromagnetic fields are influenced primarily by the surfaceproperties of a metal, low frequency (16 Hz. to 5 kHz.) fields by themass of the metal involved, and, as already indicated, the constituentsof a metal as well as its mass govern the disturbance of a magneticfield.

It is possible, for a given metal to determine the effect of introducingthe metal into electromagnetic fields oscillating at particular high andlow frequencies. A different metal will influence the electromagneticfields in a different manner. In the present invention the frequenciesof the high and low audio-frequency oscillators RFC and LFO (see FIG.1), and the frequency to which the high and low audio-frequencyreceivers I-IFR and LFR are tuned, are chosen so that there will be astrong, predictable disturbance when a gun is brought within the regionunder surveillance. This region is within the influence of transmittingcoils l, 2 and receiving coils 3, 4. Coils l, 2 are driven by theoscillators HFO and LFO, through amplifiers Al and A2 respectively. Apulse generator PG provides D.C. pulses which are amplified by amplifierA3 and applied to coil 5 to provide a magnetic field within the regionunder surveillance. Another coil 6, coupled to a magnetic flux meter 7,is used to monitor the magnetic field within the region. The outputsignals from receivers I-IFR and LFR, shown here also used to actuateswitches S1 and S2, together with the output signal from magnetic fluxmeter 7 (which may also include a switching device), are fed into logiccircuit 8. Here the signals are compared with standard signals. Theresult of the comparison is the production of an output signal from thelogic circuit if at least two of its input signals are found tocorrespond to the presence of metal of the type the apparatus isdesigned to detect. This output signal from the logic circuit actuatesan alarm 9. Switch S3 switches coil 6 out of the circuit when a D.C.pulse is applied to coil 5, and upon removal of the pulse from coil 5switches coil 6 into the circuit. In this way a signal is obtained influx meter 7 only if there is a residual field in the regioninvestigated, which will only be the case when the region contains amaterial exhibiting remanence which includes steel which has beensubjected to a magnetic field.

All firearms are constructed of a particular metal a high carbon contentsteel. However, the range of metals over which the system will operatewill be determinable by adjusting the sensitivity of the receivers andthe logic circuit.

Since the invention is suitable for use in a bank, the embodiments ofFIGS. 2 and 3 will be assumed to be installations in a bank, although itis stressed that this is not to be regarded as a limitation of theapplicability of the invention.

In its preferred form, the alarm system of the present invention isincorporated into a revolving door which permits a single person only toenter the bank. The door of FIGS. 2A and 2B, which will be assumed torotate clockwise, comprises four panels constructed of a bullet proofperspex or plastics material, such as Plexiglass or Oroglass (trademarks) produced by Rohm & Haas Company, or other polycarbonatetransparent plastics materials.

A bullet-proof glass could be used in the construction of the door butsuch doors would suffer from certain disadvantages, among which are:

i. protective glass doors are very heavy, weighing typically 200 Kg. persection, i.e., 800 Kg. for a revolving unit;

ii. the glass required to make such doors, being about 2 to 3 cms.thick, is not readily available in all countries and has, in suchcountries, to be imported, thus increasing the cost of such a door;

iii. the protective glass has to be cut carefully to prevent crackingwhich would give rise to a loss in the protective feature of the glass;

iv. replacement of a panel of glass in the event of damage is adifficult and costly process for the reasons given above; and

v. if a metal frame surrounds each glass panel, this in terferes withthedetection coils of the alarm system.

The bullet-proof panels are mounted on a central member 12 which, inthis specification, will be termed a spindle. Typically, the panels 1will be slotted into grooves in spindle 12 although, of course, othermethods of joining the panels 10 to spindle 12 can be used. The lowerand upper ends of spindle. 12 are located rotatively by non metallicbearings (not shown) of any suitable known form.

' The panels 10, which, it will be presumed, are of perspex, willtypically be 2 to 3 cms. thick. This is a suffi? cient thickness toprevent penetration of a bullet fired from a 9 mm Luger pistol. Being ofperspex, the panels are much lighter than glass, are less expensive thanglass, and there is no problem in cutting the panels to size.Furthermore, such panels need no metal frame, indeed, they need no frameat all though one may be provided for aesthetic appeal, in which case itwill be nonmetallic.

At least one of the side walls 11A, 11B of the door (the left hand onein FIGS. 2A and 2B) is constructed of a bullet-proof material but itneed not be transparent. Within this side wall an opening 13 is formedwhich may have a shute 14 leading away from it. Another preferredfeature is to have wall 1 1B on the exit side of the revolving doorshortened compared to the corresponding wall of a conventional revolvingdoor by an amount (typically one-third meter) sufficient to allow aperson leaving the bank at the same time as a would-be robber isentering the building to step out into the street and not be trappedalongside the armed entrant. No modification of the wall of the buildingwill normally be required to incorporate this feature of the wall 1 1B.

The wall 11A, however, should be lengthened, compared with thecorresponding wall on a conventional revolving door arrangement on theinterior side of the doorway to prevent an armed intruder, duly trapped,shooting into the bank through the gap between the wall 11A and theinnermost panel 10 stopped with its edge adjacent the wall 11A. Walls11A and 11B are typically of shot-proof fiberglass or are made of othersuitable non-metallic material.

One or-more loudspeakers 15 may be located above or within side wall 11Aso that a suitable message may be relayed to a person trapped in thedoor in the manner to be described below.

The transmitting and receiving coils (not shown) of the invention arearranged to produce signals and be responsive to signals within theregion being monitored, in this instance the way in to the bank. Onelocation for these coils is around the entrance as a whole, but a morepreferred arrangement is to have the coils formed in a substantiallyvertical plane, passing behind the wall 11A, across the top of half ofthe entrance, down the center of spindle l2 and underneath the floor ofthe entrance. The coils are preferably located in tubing of plasticsmaterial. The receiving coils may be contained in the same tubing or besimilarly mounted alongside the transmitting coils. An alternative, andalso practical, arrangement is to have the receiving coils for the highand low audio-frequency signals located at right angles to thetransmitting coils, in

which case they will preferably be housed in tubing which has verticalportions behind wall 11A and horizontal portions across the top andbottom thereof.

The oscillators, tuned receivers, logic circuit and alarm components maybe located remote from the door itself. There will generally be includedin the door arrangement microswitches, photo-electric cell arrangements,proximity switches 16 or the like located atop the door panels or walls.These components are used in a conventional manner to indicate theposition of the door, which information is used to ensure that the dooris locked in a required position when the alarm is actuated.

The complete alarm system will preferably be DC operated, using atransformer and rectifier arrangement to obtain the power from themains. A battery source, however, will usually be included, withautomatic switching to the battery power should the mains source beinterrupted. Such no-break power supply systems are already known and donot constitute an essential feature of the present invention, so willnot be described here.

When a person carrying a revolver or other object constructed of highcarbon content steel passes into the revolving door, he enters theregion monitored by the alarm system. The carried object disturbs thesignals being received by the three receiving coils, which results in achange of their output signals to the logic circuit. The altered outputsignals do not correlate with the normal safe comparison result in thelogic circuit and if two of the three outputs has changed by a degreecorresponding to the presence of gun material, the alarm is actuated byan output signal generated by the logic circuit.

Introduction of other metals into the monitored region will usuallycause some change in the outputs of the receivers but the disturbance ofthe signal will not correspond to the disturbance expected from gunmetal and consequently the logic circuit will produce no alarm signal.If required, a signal can be derived from the logic circuit to indicatethe presence of another metal in the monitored region by includingadditional comparison circuits in the logic circuit. Comparison circuitsand logic components are known to those skilled in the electronics fieldand therefore do not need to be described here.

The high audio-frequency signal can be varied between 5 kHz. and kHz. Alow audio-frequency signal of 1.75 kl-lz. has been found satisfactory inpractice, but this signal can be varied from 16 Hz. to 5 kHz. DC pulsesmay be produced at a rate of from one to 16 pulses per second by thepulse generator. The high audio-frequency signal should not be a simplelow harmonic of the low frequency signal.

Various circuits can be used to ensure that the alarm is triggered. Twosuch circuits will be briefly discussed here.

An audio-frequency signal from one of the receiving coils 3, 4 is fedinto one arm of a differential amplifier, while a sample of thetransmitted signal is fed into the other arm. The differencesignal-obtained from the differential amplifier, further amplified ifnecessary, is integrated and applied to the base of a transistor. If theintegrated signal is sufficiently large, it renders the transistorconducting. The act of going conducting causes a voltage increase in thecollector or emitter circuit of the transistor, which voltage increaseis applied to the gate of a thyristor or silicon controlled rectifier toswitch it on. Power through the thyristor or S.C.R. is used to actuate arelay or other switching device. A similar circuit is provided for theother receiving coil.

The circuit responsive to the magnetic field perturbation comprises anamplifier to amplify any signal produced by the flux meter 7 upondetection of a residual magnetic field when the DC pulses to coil 5 havebeen removed. This amplified signal is integrated and applied to thebase of a transistor in the same way as in the circuit described in thepreceding paragraph. The rest of the circuit is the same as thatdescribed for receiving coils 3 and 4.

If at least two of the three relays or other switching devices areactuated, the alarm is given.

A suitable logic circuit comprises three AND-gates, into each of whichis fed two signals derived in the same way as the power which is appliedto the relays in the circuits described above, except that in the caseof receiving coils 3 and 4 the received signal is compared with astandard signal in the difierential amplifiers. The first AND-gate willhave signals derived from coils 3 and 4, the second from coils 4 and 6,while the third AND-gate will be responsive to signals derived fromcoils 3 and 6.

Actuation of the alarm can have a multitude of consequences. One ofthese will almost certainly be the locking of the revolving door in aposition to trap the armed person in the quadrant of the door bounded bywall 1 1A and two panels of the door itself. As indicated above, amessage can be transmitted to the entrapped person to advise him of thesituation and that if he passes the alarm initiating metal objectthrough the opening 13 and away from the alarm detection area he may, inthe absence of the maintenance of the alarm by other metal objects inhis possession, be released at the discretion of an officer inside thebank.

Other known effects of raising an alarm, for example,

taking photographs, flashing lights, sounding bells or sirens, closingdoors and the like, either nearby or in a remote place, can beincorporated into the system as required.

An alternative arrangement of side walls of a revolving door is witheach wall extended at both ends in the same way as the innermost part ofwall 11A is extended. With this arrangement the revolving door canrotate in both directions, though the monitored region must cover bothsides of the doorway. To avoid the possibility of a bank customer beingtrapped in the opposite quadrant to one containing an armed entrant, asmall sliding doorway can be incorporated into each side wall. Thesliding doorway in the wall opposite the side where an armed person isentering can be arranged to open automatically when the revolving doorstops to permit any innocent person to escape to (for example) thestreet.

Another advantage of this alternative arrangement is that in the event aperson enters the bank and uses a bomb device constructed of a materialnot detectable by the present invention to successfully rob the bank,

an officer inside the bank can operate a manual control to trap thethief in the revolving door as he leaves.

Some places, however, are unable, by reasons of their size or otherwise,to have a revolving door included in their entrance. FIG. 3 shows howthe present invention is applied to a common door arrangement. Thetransmitting and receiving coils are suitably located in, for example,the ceiling and floor in front of the door, or in a wall or two wallsextending in front of but beside the door (see FIG. 3), or both sets ofcoils are located in the ceiling or the floor. When a person enters themutually sensitive region of the coils, if that person carries a gun, analarm is actuated and the door is locked so that it cannot be opened bythe person setting off the alarm. Appropriate action can then be takenby a person alerted by the alarm.

It will be appreciated that the invention is applicable to slidingdoors, swing doors, double doors, and conventional hinged doors. It isalso applicable to windows and gates in fact, to any form of entrancewhich can be closed.

Where one or more of the receiving coils is located in the ceilingabove, or the floor beneath, a region being monitored, two receivingcoils and receivers can be used, the second receiver coil being locatedin the alternative location to the first coil. Such an arrangement isschematically illustrated in FIG. 6, where receiving coils 21 and 22 arelocated on either side of a region under surveillance. While thiscorresponds directly to the situation where a door in a corridor or hallis con- 7 trolled by an alarm system according to the present invention,a parallel set of circumstances pertains to floor and ceiling receivercoil arrangements. The advantage of such double coil arrangements isthat the sensitivity of the alarm system can be more directly controlledto respond onlyto articles of a particular metal having a metal massgreater than a predetermined minimum value. Referring now to FIG. 6, andtaking as an example the detection of a gun weighing 500 grams,receiving coils 21 and 22 are connected to relays R1 and R2respectively. Since a firearm usually weighs at least 500 grns a metalmass of 500 grams is placed at point H and the sensitivity of the coil21 is adjusted so that it produces a signal just sufficient to triggerrelay R1. The same procedure is repeated for coil 22 and relay R2 withthe metal mass at point F. It now follows that any metal massof 500 gmsor heavier brought between coils 21 and 22 will trigger both relay R1and relay R2. In fact, metal masses less than 16 ounces will be detectedat various points other than points F and H, for example, a metal massof 125 grams will be detected at the midpoint G of a line joining pointsF and H. Metal masses between 4 ounces and 16 ounces will be detectablein various regions extending outwards from point C towards points A andB, the size of the region depending on the mass of metal. This is aconsequence of the square law detection phenomenum, the relevantquantity being mass of metal (distance from coil) To provide therequired detection level, only when relays R1 and R2 are both activatedwill the alarm system be activated. As a consequence, however, any metalmass less than I25 grams will not cause activation of the alarm at all.

It is an advantage to have all the electronic circuits and the coils ata constant temperature, for as the temperature of a coil orsemiconductor component changes, so does its electrical characteristics.It is possible for a temperature change to cause the high frequencyresonance become a harmonic of the low frequency signal. Maintenance ofa constant temperature is one of the reasons why the coils have beendescribed as enclosed in non-metallic tubes in the above embodiments ofthe invention. Air at a constant temperature is circulated around thevarious circuit components and through the tubes.

For use in the invention, a constant temperature controller has beendeveloped. A circuit diagram of this constant temperature controller isgiven in FIG.

This temperature controller regulates the supply of power to a heater insuch a manner that, as the temperature of the air being controlled risesto its required value, the power supplied is reduced, and at the momentthe required temperature of the air is reached, the power to the heateris removed. If the temperature of the air falls below its requiredvalue, then heat is applied in an amount dependent upon the differencebetween the air and the required temperatures. If the temperature of theair falls considerably then a relatively large amount of power isapplied to the heater, and as the temperature of the air approaches itsrequired value again, the heat is reduced, gradually, as alreadyindicated, until there is zero heating at the moment the requiredtemperature is attained.

Thus the temperature controller regulates the heat supplied to maintaina body at a steady, required temperature. It comprises a heater, meansfor sampling the temperature of the air, the sampling means varying aparameter thereof in response to changes in the air temperature, andmeans dependent upon the variation of said parameter to increase thesupply of heat to the air as the difference between the actualtemperature of the air and a required temperature thereof increases, andto decrease the supply of energy to the air as said differencedecreases, and to supply no energy to the air when the air is at therequired temperature.

In the temperature controller illustrated, the sampling means is athermistor, the-parameter which varies being the resistance of thethermistor, the means dependent upon the variations of the parameterisan amplifier and trigger generator, and the heater is controlled bypower supplied through silicon controlled rectifiers or thyristors.

Referring now to P10. 5, the D.C. power for-the controller is derived,in the absence of a D.C. power supply, from the A.C. mains 'supply bytransformer- TF1 and the rectifier bridge comprising diodes D1, D2, D3and D4. Zener diode D5 provides a voltage regulating element. The A.C.mains supply is also fed, via thyristors or silicon controlledrectifiers Q1 and O2 to a load, in this case a heater.

A thermistor TH is suitably located to detect the temperature of the airand is included in the base circuit of transistor TR4 so that variationof the resistance of the thermistor TH causes a variation of the voltageapplied to the base of transistor TR4. Also in the base circuit oftransistor TR4 is variable resistor R14, the adjustment of which alsohas an effect on the voltage applied to the base of transistor TR4. Thepre-set value of variable resistor R14 is chosen according to therequired temperature of the air.

The unit A, shown within dashed lines, is a trigger unit, typicallytrigger module No. MYSOOl produced by Mullard Limited, the details ofwhich are well known to those skilled in the art. A brief description,however, is not out of place here. A potential divider comprisingresistors R2 and R3 provides a reference voltage to the base oftransistor TRl. Capacitor C2, being initially uncharged, holdstransistor TR1 cut off. When the voltage across capacitor C2 exceedsthat on the base of transistor TRl, due to a change in the externalvoltage at point P, transistor TRl starts to conduct. Positive feedbackis supplied through transformer T2 and transistor TR1 bottoms, capacitorC2 discharging through the primary of the transformer T2. As a result ofthis action, pulses appear between terminals J and K and terminals L andM of the trigger module.

The voltage at P is dependent upon the state of transistor TR2, which iscontrolled by the base voltage applied to it as a result of currentthrough resistors R12 and R11 and transistor TR3. Transistor TR3,however, is controlled by transistor TR4, which in turn is, as alreadyindicated, governed by the values of resistance of thermistor TH andvariable resistor R14. Consequently the resistance of thermistor THgoverns the voltage at P, which determines when pulses are generated atterminals J and K and terminals L and M of the trigger circuit.

These pulses are applied to the gates of silicon controlled rectifiers(S.C.R.s) or thyristors Q1 and Q2. Power is only passed by S.C.R.s Q1and Q2 when a voltage is applied to the gates, so the quantity of powerpassed will depend, in the case of an AC. power supply, upon the phaseof the pulses applied to the gates relative to the AC. mains supply.When the pulses are applied at or near the beginning of a mains cycle,or at or near a point where the mains voltage passes through a zeropoint, very little power will be passed by S.C.R.s Q1 and Q2. The lengthof the pulse governs the time Q1 and Q2 are conducting. If the pulse isapplied at or near the peak value of the mains voltage, then a maximumquantity of power is passed by S.C.R.s Q1 and Q2 for the duration of thepulse. If the apparatus is powered by a DC. supply, only the duration ofthe pulses can be used to control the heat applied by the heater.

It will be clear to those persons skilled in the art that a variation ofthe resistance of thermistor TH, by controlling the voltage at P, causesa variation of the time of applying pulses, and their length and hencethe quantity of power passed by S.C.R.s Q1 and Q2. Resistance R14 can beset so that when the air is at the required temperature, no pulses aregenerated by the trigger module, but the resistance of thermistor THbecomes influential and pulses are generated immediately the temperatureof the air drops below the required temperature.

The arrangement of resistors R8 and R9, with capacitor C3, is forsuppression purposes. The control of temperature by this meansrepresents a substantial improvement on the prior art.

The controller is, in fact, applicable to other situations than merelythe present application. To illustrate its usefulness, in a 50 C range,using an E208AEP/22K thermistor, a circuit sensitive to 0.000 1 C can beconstructed, so the body temperature of a liquid can be controlled toi0.05 C quite easily. In a practical circuit constructed to test theinvention, a pulse rise time (to a value of 0.25 to 0.3 volt) of 4microseconds was obtained, the pulse duration was 10 microseconds andthe conduction angle range was between 20 and 150. This represents acontrol range of power into the heater of from 3 to 87 percent of themaximum possible power.

It was indicated early in this specification that the invention can beused in airport security systems. FIG. 4 illustrates one way in whichthe invention an be so used.

The illustrated embodiment of the invention comprises a first corridorhaving a first revolving door located therein, a second revolving dooradjacent said first revolving door and located in or at the end of asecond corridor, under normal circumstances said second revolving doorbeing locked in a position preventing entry to said second corridor andsaid first revolving door being rotatable to permit a person to travelalong said first corridor, an alarm system according to the presentinvention located in or near the swept region of said first revolvingdoor at or near the entry point thereof, said system being operative,upon detection of a gun-metal object, to allow said first revolving doorto rotate to a position where the person carrying the metal objectcannot proceed in either direction along said first corridor and to locksaid first revolving door in that position while rendering said secondrevolving door rotatable, whereby access to said second corridor isavailable to said person.

Passengers about to board an aircraft leave the passenger lounge 30 andproceed along a first corridor 31 to their aircraft. A ticket checkingpoint 33 may be included in or near the corridor 31, but this is not anessential feature of the invention. A revolving door 34 is located incorridor 31, and alongside it is a second revolving door 35. Revolvingdoor 35 is at the end of a second corridor 36 and under normalcircumstances is held locked in the position shown in the drawing, thuspreventing access to corridor 36 from corridor 31 through revolving door35. Revolving door 34 is rotatable and as long as the alarm system isnot actuated, passengers can pass through revolving door 34 and continuealong corridor 31. Corridor 36 need not be parallel to corridor 31 Inthe dashed-outline region D is located, in the ceiling and floor of thecorridor, sets of transmitting and receiving coils of the type referredto earlier in this specification. A person carrying a gun or metalobject made out of gun metal who walks between the sets of coils willinfluence the coupling between the transmitting and receiving coils andwill trigger an alarm. In the present invention, along with anyconventional alarm response (such as flashing a light, taking aphotograph, recording events photographed by a T.V. camera), thetriggering of the alarm causes the lock to be removed from revolvingdoor 35 and causes revolving door 34 to become locked to prevent theperson triggering the alarm from continuing along'corridor 31, i.e., tobecome locked in the position shown in the drawing with the person (apotential hijacker) required to use revolving door 35 and enter corridor36 to leave the revolving door region. Once the revolving door 36 hasrotated a quarter turn and the potential hijacker has left the regionswept by revolving door 34, revolving door 36 is again locked, revolvingdoor 34 is released and travel along corridor 31 can be resumed.

It is preferable, to minimize the disturbance to the flow of passengersalong corridor 31, to have revolving door 34 slow down to its lockingposition rather than stop abruptly. To facilitate this, wall 37 projectsinto corridor 31. The slowing down of revolving door 34 when the alarmis triggered begins when the potential hijacker is effectively held inrevolving door 34 by wall 37. At this point, or shortly thereafter,suitable messages can be relayed, for example by loud-speakers, topassengers in corridor 31 approaching revolving door 34.

A portable version of this embodiment of the invention can also beconstructed for use with individual aircraft, with corridors 31 and 36becoming entry and non-entry paths for the aircraft.

In all embodiments of the invention, it is preferred to have oscillatorsHFO and LIFO in the form of oscillators having their frequenciescontrolled by tuning forks. Another preferred feature is the continuouschecking that the power supply, oscillators, pulse generator, coils,receivers and flux meter are all functioning. if one integer of theinvention should fail, then a different type of alarm will be given toadvise the malfunction to an operator. An indication of the mass ofsteel detected by the alarm system may also be included, such indicationtypically including a meter approximately calibrated for small pistols,large pistols, small rifles and largerfirearms.

- it is to be understood that embodiments of the invention have beendescribed in this specification andvariations can be made and. still bewithin the scope of the invention, as defined by the claims. Forexample, use of microelectronic techniques may, in thefuture, permit analarm system incorporating the present invention to be made of a sizesufficient to be carried by one person.

lclaim:- I l. A metal detection and alarm system comprising a regionthrough or overv which a person carrying a metallic object must pass; afirst and second oscillator coupled, respectively, to a first and secondtransmitting coil, each: said coil being adapted to produce signalswithin said region; a first and second receiving coil each responsivetosignals within said region and coupled to a first and secondreceiver,respectively; said first oscillator being adapted to produce a lowaudiov frequency signal, said second oscillator being adapted to producea high audio-frequency signal, said first and second receivers beingtuned to the frequency of the signals of said first and secondoscillators, respectively; a-pulse generator adapted to, supply D.C.pulses to a magnetic field producing coil to produce a magnetic fieldwithin said region, and a third receiving coil couswitching devicesproviding an output signal on actua-- tion of at least two of saidswitching devices, said signal from said logic circuit or said switchingdevices causing actuation of an alarm.

2. A system as claimed in claim 1, in which said low audio-frequency isa frequency in the range l6 Hz. to 5,000 Hz., said high. audio-frequencyis in the range 5,000 Hz. to 20,000 Hz., and said D.C. pulses aregenerated at a rate of from one to 16 pulses per second.

3. A system as claimed in claim 2, in which said third receiving coil isdisconnected from said magnetic flux detector while one of said D.C.pulses is applied to said magnetic field producing coil and isre-connected to said magnetic flux detector on the termination of thepulse.

4; A system as claimed in claim gion comprises a doorway.

5. A system as claimed in claim 4 in which said doorway comprises anentrance including a revolving door 3, in which said rehavingtransparent, bullet-proof panels, said revolving door being locatedbetween curved side walls, actuasaid side walls.

tion of said alarm being effective to lock said revolving door in aposition where a person carrying said metallic object is trapped betweentwo of said panels and one of 6; A system as claimed in claim 5, inwhich said revolving door can rotate in a single direction and the sidewall alongside which a person must pass on'leaving through side doorwayis terminated short of its normal termination point to enable a personlocked in the o posite quadrant to a person rapped on actuation of saidalarm to leave theproximity of said doorway.

7. A system as claimed in claim 5 in which said side walls each extendaround more than one quarter the circumference of the area swept out bysaidrevolving door and each contain a smalldoor therein which isopenable when a person is trapped inthe quadrant including said otherside wall.

8. A system as claimed in claim 6, in'which theside wall alongside whicha person entering through said doorway must pass has therein a smallopenable region through which a person trapped by actuation of saidalarm can pass the metal object which has actuated said alarm. v v

9. A system as claimed'in claim 7, in which said side walls each have asmall openable region through which a person trapped by actuation ofsaid alarm can pass the metal object which has actuated the alarm.

10. A system as claimed in claim 5, in which said revolving door andsaid side walls are non-metallic.

11. A system as claimed in claim 5, in which at least one of said coilsis formed in a vertical plane and has one side located in the center ofa spindle upon which the panels of said revolving door are mounted.

12. A system as claimed in claim 1 in which said coils, saidoscillators, said pulse generator and said magnetic flux detector areall located within a constant temperature enclosure.

13. A system as claimed in claim 1, in which said region includes afirst revolving door located in a first passageway, a second revolvingdoor being located adjacent said first revolving door in or at the endof a second passageway, said second revolving door being normallylocked, actuation of said alarm causing said first revolving door to belocked and said second revolving door to be unlocked so that a personcarrying said metal object is forced to enter said second passageway.

14. A system as claimed in claim 1, in which said metallic objectcomprises an object made of high carbon content steel of the type ofwhich firearms are made.

t i i i'

1. A metal detection and alarm system comprising a region through orover which a person carrying a metallic object must pass; a first andsecond oscillator coupled, respectively, to a first and secondtransmitting coil, each said coil being adapted to produce signalswithin said region; a first and second receiving coil each responsive tosignals within said region and coupled to a first and second receiver,respectively; said first oscillator being adapted to produce a lowaudio-frequency signal, said second oscillator being adapted to producea high audiofrequency signal, said first and second receivers beingtuned to the frequency of the signals of said first and secondoscillators, respectively; a pulse generator adapted to supply D.C.pulses to a magnetic field producing coil to produce a magnetic fieldwithin said region, and a third receiving coil coupled to a magneticflux detector; each of said receivers and said magnetic flux detectorbeing adapted to produce an output signal to be either fed into a logiccircuit capable of comparing each said output signal with a respectivereference signal or used to actuate a switching device; said logiccircuit producing a signal when there is a departure of at least two ofsaid output signals from their respective reference signals or saidswitching devices providing an output signal on actuation of at leasttwo of said switching devices, said signal from said logic circuit orsaid switching devices causing actuation of an alarm.
 2. A system asclaimed in claim 1, in which said low audio-frequency is a frequency inthe range 16 Hz. to 5,000 Hz., said high audio-frequency is in the range5,000 Hz. to 20,000 Hz., and said D.C. pulses are generated at a rate offrom one to 16 pulses per second.
 3. A system as claimed in claim 2, inwhich said third receiving coil is disconnected from said magnetic fluxdetector while one of said D.C. pulses is applied to said magnetic fieldproducing coil and is re-connected to said magnetic flux detector on thetermination of the pulse.
 4. A system as claimed in claim 3, in whichsaid region comprises a doorway.
 5. A system as claimed in claim 4 inwhich said doorway comprises an entrance including a revolving doorhaving transparent, bullet-proof panels, said revolving door beinglocated between curved side walls, actuation of said alarm beingeffective to lock said revolving door in a position where a personcarrying said metallic object is trapped between two of said panels andone of said side walls.
 6. A system as claimed in claim 5, in which saidrevolving door can rotate in a single direction and the side wallalongside which a person must pass on leaving through side doorway isterminated short of its normal termination point to enable a personlocked in the opposite quadrant to a person trapped on actuation of saidalarm to leave the proximity of said doorway.
 7. A system as claimed inclaim 5 in which said side walls each extend around more than onequarter the circumference of the area swept out by said revolving doorand each contain a small door therein which is openable when a person istrapped in the quadrant including said other side wall.
 8. A system asclaimed in claim 6, in which the side wall alongside which a Personentering through said doorway must pass has therein a small openableregion through which a person trapped by actuation of said alarm canpass the metal object which has actuated said alarm.
 9. A system asclaimed in claim 7, in which said side walls each have a small openableregion through which a person trapped by actuation of said alarm canpass the metal object which has actuated the alarm.
 10. A system asclaimed in claim 5, in which said revolving door and said side walls arenon-metallic.
 11. A system as claimed in claim 5, in which at least oneof said coils is formed in a vertical plane and has one side located inthe center of a spindle upon which the panels of said revolving door aremounted.
 12. A system as claimed in claim 1 in which said coils, saidoscillators, said pulse generator and said magnetic flux detector areall located within a constant temperature enclosure.
 13. A system asclaimed in claim 1, in which said region includes a first revolving doorlocated in a first passageway, a second revolving door being locatedadjacent said first revolving door in or at the end of a secondpassageway, said second revolving door being normally locked, actuationof said alarm causing said first revolving door to be locked and saidsecond revolving door to be unlocked so that a person carrying saidmetal object is forced to enter said second passageway.
 14. A system asclaimed in claim 1, in which said metallic object comprises an objectmade of high carbon content steel of the type of which firearms aremade.